The present invention relates to the field of oriented thermoplastic films having utility in packaging and solar control applications. More particularly, the present invention relates to a coated thermoplastic film which is resistant to moisture and which exhibits enhanced adhesion to inks and to metallic coatings deposited onto the film.
Oriented thermoplastic film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a packaging material for a variety of products, including foodstuffs.
Because the surface of oriented PET film is hydrophobic, it is not readily receptive to coating or printing. In many packaging and label applications it is advantageous that the film be receptive to ordinary flexographic and rotogravure printing inks.
In an attempt to remedy this deficiency, special inks have been devised that adhere directly to the surface of PET film. However, many such inks are based on formulations or organic solvents, resinous binders and other ingredients which are costly, difficult to handle and hazardous for use in the manufacturing plant.
Another approach for improving the printability of PET film is to apply a primer coating which adheres to the film and is receptive as well to printing inks applied thereto. Examples of such primer coatings include polymeric compositions based on vinylidene chloride terpolymers such as disclosed in U.S. Pat. No. 2,698,240 or water dispersible copolyester primers such as disclosed in U.S. Pat. No. 4,252,885. The problem with this approach to enhancing ink adhesion to PET film is that the film is in many instances used as a food packaging material and consequently the relative toxicity of the primer layer becomes important. Though polyester film per se is approved as a food packaging material by most Governmental agencies, including the U.S. Food and Drug Administration, polyester film primed with certain polymeric primer coatings such as acrylics, vinylidene chloride-containing polymers and many other similar primers raises toxicity problems because of potential migration of said primer material into foodstuffs with which it is in contact.
Coated polyester films having superior ink adhesion properties are disclosed by U.S. Pat. Nos. 4,486,483 and 4,544,200 to Caines. The non-toxic coatings disclosed therein comprise alkali metal salts of unsaturated fatty acids and alkali metal salts of alkyl sulfates.
In many instances thermoplastic film coated with a metal has replaced metallic foils, such as aluminum foil, for applications such as labels and decals, solar window film, packaging films, decorative products and microfilm. Among other uses, the metallic foils serve to hinder the onset of oxidative rancidity of the products either prepared in or served with certain edible oils by preventing the passage of visible and ultraviolet light. PET, when coated with a thin metallic layer, provides superior oxygen and moisture barrier properties, along with the visible and ultraviolet light barrier properties common to metallic foil packaging applications at a greatly reduced cost. Unfortunately, the metal to film bond can fail in many applications, thereby reducing the performance of the metallized film. One approach for improving the adhesion of the metal to PET film is to heat the coated film as disclosed in British Pat. No. 1,370,893. The drawback to this method is the experimentation which must be performed in order to determine the proper temperature and time elements for this procedure before any metallized film may be produced. Another approach has been to coat polyester film with a layer comprising a random copolymer of ethylene with from 0.5 to 15 weight percent of a 3 to 6 carbon alpha-mono-olefin as disclosed in European Publication No. 0,023,389. Although PET films with this type of coating display a certain improvement in metal adhesion over uncoated PET, they also display an undesirable coating pattern.
A third approach is to coat the polyester film with a copolyester coating which enhances the metal adhesion properties of polyester film. U.S. Pat. No. 4,493,872 to Funderburk teaches a specific sulfonated copolyester coating which provides superior metal adhesive properties to oriented thermoplastic film.
The moisture resistance of the coatings applied to polyester films may be important in applications where the coated film is placed in a relatively humid or moist environment. For example, polyester films employed in food packaging may be exposed to moisture due to condensation from refrigeration and/or high humidity and temperature levels during microwave cooking of the food contained within the packaging. Unfortunately, the non-toxic, ink adhesion coatings disclosed in U.S. Patent No. 4,486,483 do not possess superior resistance to moisture.
Similarly, it is desirable that metalized film possess resistance to moisture, especially in solar control applications where condensation can readily form on the window. Unfortunately, the sulfonated copolyester metal adhesive coating disclosed in U.S. Pat. No. 4,493,872 does not possess superior resistance to moisture.